1. Field of the Invention
The microwave equalizer of the present invention relates in general to the field of microwave equalizers characterized by having a predetermined frequency dependent attenuation characteristic over a given band of operating frequencies. More particularly, the present invention relates to the art of providing such an equalizer through the use of a length of transmission line to which is coupled at least a pair of substantially identical resonant stubs and shunt impedances spaced at quarter wavelength intervals.
2. Description of the Prior Art
It is well known that most microwave devices or networks which generate, amplify or transmit electromagnetic energy in the microwave region exhibit an output whose relative amplitude or gain is frequency dependent. Thus, over a given frequency band of operation, or so-called operating band it is seen that the output is relatively greater for some frequencies than for others.
A good example of such behavior is the traveling wave tube whose somewhat peaked gain (at band midpoint) versus frequency characteristic is well known. However, the requirements of users of such devices have become quite exacting as to what constitutes acceptable operating characteristics or parameters when such devices are utilized in apparatus having increasingly stringent and involved design criteria. Consequently, one user may require the traveling wave tube to produce a flat gain versus frequency characteristic, while another may require a tapered characteristic, and another may require a certain variable characteristic. It is apparent that the difficulty and expense of redesigning the active device, here the traveling wave tube, to produce the required output characteristic effectively precludes such an option.
Hence, passive attenuation devices or networks called "equalizers" have been developed which when coupled to the output of the active microwave device, such as a traveling wave tube, can modify the output thereof over a given operating frequency band so as to produce the required gain versus frequency characteristic. Many different forms of equalizing devices are known, but all share the common feature of exhibiting an attenuation curve which, over the operating frequency band, mirrors somewhat the normal frequency versus gain characteristic of the active device in such a way that the desired frequency versus gain characteristic is obtained.
In general it is known, or can be easily derived from text book knowledge, that it is possible to fabricate a microwave equalizer utilizing the basic principle of coupling two resonant stub transmission line segments to a third transmission line through absorptive loss elements at a spacing of approximately one-quarter wavelength of the frequency within the operating frequency band at which maximum attenuation occurs. Such devices are shown in U.S. Pat. No. 3,548,344 issued Dec. 15, 1970 to Putz and U.S. Pat. No. 3,648,200, issued March 7, 1972 to Harrison et al, for example. However, the equalizers disclosed in these patents are physically large and relatively bulky, and quite likely, correspondingly relatively heavy. Further, these patents do not disclose any means by which to compensate for so-called standing waves, commonly considered in terms of the devices' voltage standing wave ratio (VSWR), which are inevitably introduced into this type of microwave equalizer through the use of the aforesaid resonant stubs.
In many applications for such microwave equalizers, as in the aerospace industry, it is readily appreciated that where weight and space are at a premium, microwave equalizers exhibiting small size and a correspondingly low weight are exceptionally valuable. Yet, to be economic, such an equalizer should not exhibit the usual hefty cost increases usually associated with the miniaturization of devices. Naturally, if a small, lightweight, low cost microwave equalizer was available with a desirable, lower voltage standing wave ratio than is normally encountered in the aforesaid class of microwave equalizers which utilize quarter wavelength spaced resonant stubs, its contribution to the art would be self-evident. Such a device is provided by the microwave equalizer of the present invention.